1. Field of the Invention
The present invention relates to rolling bearings used, e.g., with automotive vehicles, agricultural machinery, construction machinery and iron-and-steel machinery, and, more particularly, to long-life rolling bearings required for use in transmissions and engines.
2. Description of the Related Art
A rolling bearing is typically in a severe use under high surface pressure so as to experience a repeated shearing stress. In order to endure the repeated shearing stress and secure long rolling fatigue life (hereinafter referred to as life), a rolling bearing which is made of high-carbon chromium steel (e.g., JIS-SUJ2), then quenched and tempered so as to have a 58-64 H.sub.R C hardness has been used. On the other hand, there is a prior-art rolling bearing which is made of case hardened steel in order to increase its life. This prior-art case hardening steel rolling bearing has been made of a low-carbon case hardened steel of a good hardenability, e.g., JIS-SCR420H, -SCM420H, -SAE6820H or -SAE432OH, since a hardness curve of the case hardened steel rolling bearing must be determined in accordance with an internal shearing stress distribution created by a contact surface pressure. Then, this case hardening steel rolling bearing has been carburized or carbonitrided, quenched and tempered and thus has secured a required life so that the races of the case hardened steel rolling bearing have 58-64 H.sub.R C surface or case hardnesses and 30-48 H.sub.R C core hardnesses.
The load on a machine using a rolling bearing has increased and a rotational speed of the machine has increased so that service conditions of the rolling bearing has become more severe, and the rolling bearing has been used under a quasi-high temperature which has caused the following problems:
First, an increase in the service temperature of a rolling bearing decreases the hardness of the rolling bearing resulting in a plastic deformation in the rolling bearing which decreases the life of the rolling bearing. Second, using a rolling bearing at a high temperature can break an oil film between the races and rolling elements of the rolling bearing so that boundary lubrication fails to occur resulting in an acceleration in the wearing in the rolling contact elements of the rolling bearing. Third, some of retained austenite present in an element of the rolling bearing transforms to martensite at a high temperature so that the size of the element changes thereby damaging the dimensional stability of the rolling bearing.
Thus, in order to preferentially secure the dimensional stability of the rolling bearing used at a quasi-high temperature, a high-temperature tempered type of high-carbon chromium bearing steel (JIS-SUJ2) in which the high-carbon chromium bearing steel has been high-temperature tempered so as to previously transform retained austenite in the rolling contact elements of the rolling bearing to martensite to thereby increase the dimensional stability of JIS-SUJ2 has been conventionally provided.
Precipitation-hardened steel, such as M50, high-temperature bearing, high-speed steel with Cr, Mo and V, has been provided as a material for a high-temperature rolling contact element in addition to the high-temperature tempered type of JIS-SUJ2 steel. M50 is a high-temperature bearing, high-speed steel which is high-temperature tempered to precipitate an alloy carbide so as to provide a sufficient high-temperature service strength to a rolling bearing.
In addition, in order to increase the life of a rolling bearing, the service temperature of which can be a quasi-high temperature to high temperature, a wear-resistant carburized steel of Unexamined Japanese patent application publication No. SHO 52-130415 or a case hardened steel of Unexamined Japanese patent application publication No. SHO 57-164977 can alternatively be used as the material for the rolling contact element.
The high-temperature tempered type of JIS-SUJ2 steel increases the dimensional stability while the high-temperature tempering decreases the hardness of the high-temperature tempered type of JIS-SUJ2 steel to thereby decrease the life of a corresponding rolling bearing due to a plastic deformation in a rolling contact element made of a high-temperature tempered type of JIS-SUJ2 steel. In addition, an insufficient wear resistance of the high-temperature tempered type of JIS-SUJ2 remarkably accelerates wear in the rolling bearing under a boundary lubrication during a high temperature service.
In addition, since the concentration of carbon in M50 precipitation-hardened steel is high and M50 precipitation-hardened steel in the stage of a material for a rolling contact element of a rolling bearing has macrocarbides of Cr, Mo and V, the workability of M50 in pretreatment is poor. On the other hand, the macrocarbides cause a stress concentration therearound which can result in a flaking from the place of the stress concentration to thereby decrease the life of a corresponding rolling bearing with a rolling contact element made of M50. In addition, the workability of M50 in preworking of the rolling bearing with a rolling contact element made of M50 is poor. Thus, M50 must be solution heat treated at a high temperature (about 1,100.degree. C.) in order to dissolve the macrocarbides into the matrix of M50 and then particularly heat treated in order to refine the resulting carbides. This requires a separate heat treatment step and decreases the productivity of the heat treatment.
The wear-resistant carburized steel of the Unexamined Japanese patent application publication No. SHO 52-130415 has large contents of Mo and Cr so that the sizes of the carbides of Mo and Cr can be large.
The case hardened steel of the Unexamined patent application publication No. SHO 57-164977 has a large content of carbon so that the size of a carbide can be large and, on the other hand, the content of retained austenite in the case hardened races or rolling elements is great so that the dimensional stability of this case hardened steel is decreased.
Therefore, in order to solve the above problems, a primary object of the present invention is to provide a rolling contact element steel which will not soften even at high temperature and the dimensional stability at high temperature and the wear-resistance of which are superior and which can produce a fine carbide without the need for a separate heat treatment. A secondary object of the present invention is to provide a rolling bearing which is made of the inventive rolling contact element steel and has a long life even under use at a high temperature.